Sometimes one will hear of a photo being reprinted, redistributed, or otherwise used without specifically being licensed or having permission granted. Often the defense is called "fair use". What does this mean from a legal standpoint?
Fair use describes a clause in copyright law that permits use of limited parts of copyrighted materials without permission in restricted situations. Usually for commentary, criticism, reporting, etc.
Fair use normally requires that only a small percentage of the whole work be used (for example a quote of a few lines from a book), so with regards to photo's you may find fair use can only be applied when dealing with collections of copyrighted images, or if your reproduction is of significantly reduced quality.
[I am not a lawyer and fair use clauses will vary between jurisdictions so it is best to check with the original author of a work if you are unsure.]
Wikipeida has a more detailed article on fair use
Fair use is a construct in US law, that allows you to use copyrighted works without permission of copyright holder, under some specific circumstances (usually commentary, teaching, news reporting or similar). For example, English Wikipedia can use a copyrighted "Tank man" photo in its article without getting a special license. See image description for long legalese that explains why exactly is it possible in this case, and Fair use article for explanation of details.